This page explains the main functionalities of GSM networks, including mobile-originated and mobile-terminated voice calls, mobility, handover, SMS, and international roaming.
A call originating from a mobile device (MS) in the GSM network is routed through the core network to the destination party. To connect to the GSM network, an MS first connects to a radio network, which is in charge of handling messaging between devices and the core network. An MS is constantly communicating with the Base Station Subsystem of the radio network to send and receive signals. The steps to establishing a GSM voice call are illustrated below.
- to initiate a call, the MS sends a request for radio resource allocation to the BSS, which mediates further connection to the Mobile Switching Center (MSC). The BSS assigns the MS a channel with a given frequency and time slot, which constitutes the communication route between the MS and the BSS.
- once the MS confirms the established channel, the BSS can initiate the connection to the MSC.
- the next step once the MS has been connected to the network is to authenticate the subscriber. This can be done using the IMSI number stored in the SIM card, which allows the Authentication Center in the core network to verify the subscriber's identity. Once this has been done, the MS and the MSC can start communicating.
- in order to initiate the call setup, the MSC verifies that the requested service is allowed for the subscriber. This information is available in the Virtual Location Register, which maintains temporary subscriber data (location, preferences, allowed services). Once the VLR confirms the service requested by the originating MS, the MSC starts the call setup.
- for the call to take place, the MSC allocates a voice channel between the MSC and the BSS. The BSS notifies the MS about the change to voice mode, and the MS returns a confirmation message. The MSC routes the call to the dialed number. When the call is received in the PSTN, the MSC is notified that the called subscriber is being alerted, at which point the originating MS receives a ring notification.
To make sure the data sent over the radio network between the BSS and MS/MSC is secured, the MSC initiates a ciphering procedure which is transmitted to the BTS, which in turn forwards the message to the MS. The MS enables ciphering towards the BTS; as the BTS starts receiving ciphered data, it will start the ciphered transmission of information, finalizing the encryption procedure.
To disconnect the call by either party, a disconnect message is sent to the MSC, which releases the communication channels created with the PSTN and the BSS.
A terminating call (showed on the right side of the diagram above) in the GSM network is a call received on a mobile device. For an MT to be placed, it is necessary to locate the network to which the called subscriber is currently connected, and, once this is done, to route the call towards the destination MS.
- when a call is placed from the PSTN towards a given phone number, the PSTN uses the information in the phone number (country and if available, operator) to locate gateway MSC leading to the MSC where the subscriber is registered.
- the GMSC can request information about the subscriber's core network and current location by interrogating the HLR (Home Location Register).
- the HLR constantly updates locations of the MS stored in the VLRs of the networks the MS visits. In the HLR, the subscriber MSISDN (phone number) is associated with the IMSI number of the SIM card, which was used to authenticate the subscriber in the visited network as they registered. Since authentication is communicated to the MSC, the HLR is aware of the visited MSC/VLR of the MS at a given time.
- In order for the GMSC to pass the call to the MSC, the HLR asks a temporary roaming phone number from the MSC (Mobile Station Roaming Number - MSRN).
- the MSRN is sent back from the HLR to the GMSC.
- the GMSC forward the call to the MSC using the assigned MSRN.
- having received the call, the MSC pages all the BSCs in the area that it serves.
- the BSC, in turn, page the BTSs assigned to them.
- the called MS responds to the paging from the BTS, asking to establish a radio channel to the BTS.
- the response is forwarded to the MSC, which, once notified, authenticates the MS and initiates the ciphering of the call using the same procedure as in MO calls.
- when the MSC sends back to the radio network the call confirmation message, the called MS starts to ring.
At the other end, the MSC notifies the GMSC, which notifies the PSTN that the destination number is being alerted.